(a) Field of the Invention
The invention relates to a charge pump circuit, particularly to a charge pump circuit control system.
(b) Brief Description of the Related Art
FIG. 1A illustrates a traditional charge pump circuit 10. The charge pump circuit 10 comprises a booster 11, a transfer gate 12, and a capacitor CVPP. The booster 11 comprises a capacitor CBST and a MOS transistor MN1. The transfer gate 12 is implemented by a MOS transistor MN2. FIG. 1B illustrates the equivalent circuit 10′ of the charge pump circuit 10 where the transistors MN1 and MN2 are represented by equivalent switches SW1 and SW2, respectively. The charge pump circuit 10′ operates in three different states, which are the precharge state, the charge sharing state, and the off state. During the precharge state, the switch SW1 is turned on, the switch SW2 is turned off, and the voltage boosting signal BST is at the low voltage level 0 so that the voltage VDD charges the capacitor CBST and the voltage on the node N1 changes from the low voltage level VPP-VDD to the high voltage level VDD. During the charge sharing state, the switch SW1 is turned off, the switch SW2 is turned on, and the voltage boosting signal BST is at the high voltage level VDD (that is the voltage boosting signal BST provides one unit of boosting power) so as to elevate the voltage on the node N1 from VDD to 2 times VDD. The electric charge stored in the capacitor CBST is discharged through the switch SW2 to generate an output voltage VPP and thus the voltage on the node N1 changes from the high voltage level, 2 times VDD, to the low voltage level VPP. During the off state, the switch SW1 is turned off, the switch SW2 is turned off, and the charge pump circuit 10′ is idle, that is, the control voltages of MN1 and MN2 remain at the low voltage level to reduce the stress on the oxides of MN1 and MN2 so as to extend the life of the charge pump circuit itself.
As shown in FIG. 2, in order to reduce the cost of the control circuit, extend the life of the charge pump circuit, and get a more uniform output current, a charge pump circuit 20 is commonly designed to comprise two charge pump circuits 10'A and 10'B cooperating with each other. It can be understood for those who are skilled in the art that the switches SW1A and SW2B share one control signal while the switchs SW1B and SW2A share another control signal. Besides, during circuit operation, the charging and discharging operations are executed simultaneously by the two charge pump circuits 10′A and 10′B to achieve the above mention objectives.
FIG. 3A illustrates a traditional charge pump circuit control system 30. The charge pump circuit control system 30 comprises a level detector 31, a ring oscillator 32, and a charge pump circuit 33. FIG. 3B is a schematic diagram illustrating the ring oscillator 32. The ring oscillator 32 comprises a NAND gate NAND and six inverters Inv1˜Inv6. FIG. 3C is a schematic diagram illustrating a portion of the control circuit 33′ and the charge pump circuit 20 in the charge pump circuit 33. The control circuit 33′ comprises an inverter Inv and two NOR gates. The charge pump circuit control system 30 uses the charge pump circuit 20 to output an voltage VPP or VBB with a preset voltage level, uses the level detector 31 to detect the variation of the voltage VPP or VBB, and generates a control signal ENVPP according to the detection result. The ring oscillator 32 receives the control signal ENVPP to generate clock signals RO, such as ROA, ROB and so forth. The control circuit 33′ of the charge pump circuit 33 generates boosting signals BSTA and BSTB according to the control signal ENVPP and the clock signals ROA and ROB and uses the boosting signals BSTA and BSTB to control the charging and discharging operations of the charge pump circuit 20 to tune the declined voltage VPP or VBB back to the original preset voltage level.
FIG. 3D illustrates the waveforms of the various signals of the charge pump circuit control system 30 during operation, where 10′A and 10′B represent the two charge pump circuits as shown in FIG. 2, “P” indicates that the charge pump circuit 10′A or 10′B is in the precharge state, “C” indicates that the charge pump circuit 10′A or 10′B is in the charge sharing state, and “O” indicates that the charge pump circuit 10′A or 10′B is in the off state. Please refer to FIGS. 2, 3A, 3B, 3C and 3D simultaneously.
At the time T1, as shown in FIG. 3D, the level detector 31 detects that the output voltage VPP or VBB of the charge pump circuit 20 is depleted by the load and thus the VPP or VBB is lower than the preset voltage level. Thus, the level detector 31 enables the control signal ENVPP to the high voltage level 1. The NAND gate of the ring oscillator 32 receives the control signal ENVPP at the high voltage level 1 and then generates the clock signal ROA at the low voltage level 0. In addition, the inverter Inv1 is used to invert the ROA to generate the clock signal ROB at the high voltage level 1. Then, the inverter Inv of the control circuit 33′ receives the control signal ENVPP and inverts the voltage level of the control signal ENVPP to the low voltage level 0. The NOR gate NOR1 receives the clock signal ROA at the low voltage level 0 and the control signal ENVPP to generate the boosting signal BSTA at the high voltage level 1. The NOR gate NOR2 receives the clock signal ROB at the high voltage level 1 and the control signal ENVPP at the low voltage level 0 to generate the boosting signal BSTB at the low voltage level 0. Then, the charge pump circuit 10′A receives the boosting signal BSTA at the high voltage level 1 and enters the charge sharing state “C”, while the charge pump circuit 10′B receives the boosting signal BSTB at the low voltage level 0 and enters the precharge state “P” to perform the pre-charging/discharging control on the output voltage VPP or VBB.
At the time T2, the clock signal ROA changes to the high voltage level 1 and ROB changes to the low voltage level 0. Correspondingly, the boosting signals BSTA and BSTB change to the low voltage level 0 and the high voltage level 1, respectively, so that the charge pump circuits 10′A and 10′B changes to “P” and “C” states, respectively. Thereafter, the charge pump circuit 20 continuously raises the voltage to the preset voltage level, does not stop until the time T6, and then enters the off state. At the time T7, the output voltage VPP of the charge pump circuit 20 is again depleted by the load so that the control signal ENVPP is enabled and thereby the various components of the charge pump circuit control system 30 then perform the voltage boosting operation again.
As shown by the phase 1 and the phase 2 in FIG. 3D, when the voltage of the control signal ENVPP changes from the low voltage level 0 to the high voltage level 1 (that is, the charge pump circuit changes from the off state to pre-charging/discharging state), the charge pump circuit 10′A repeats the charge sharing “C” operation before the off state “O” once. Thus, the voltage boosting energy of the boosting signal BSTA is wasted. On the other hand, the charge pump circuit 10′B repeats the precharge “P” operation before the off state “O” once. The charged capacitor is charged again and therefore it causes unnecessary energy usage. In addition, as shown by the phase 3 and the phase 4 in FIG. 3D, when the voltage of the control signal ENVPP changes from the low voltage level 0 to the high voltage level 1, the charge pump circuit 10′A repeats the charge sharing “C” operation once, that is, the charge pump circuit 10′A has no additional charge to perform the charge sharing “C” operation during the phase 4. Since the charge pump circuit 10′A has not performed the precharge “P” operation to replenish the depleted charge and the boosting signal BSTA is enabled repeatedly, the boosting energy of the boosting signal BSTA is consumed. On the other hand, the charge pump circuit 10′B performs the precharge “P” operation repeatedly but does not perform the charge sharing “C” operation. Therefore, unnecessary energy is consumed so that the efficiency of the charge pump circuit 20 is equal to zero.